Non-enzymic reaction between lysine in proteins and reducing sugars, leading to a brown colour. A similar reaction occurs in the glycation of proteins in diabetes mellitus. The first step in the reaction is the formation of a Schiff base (aldimine) between the aldehyde group of the sugar and the ε- amino group of lysine, followed by isomerisation (Amador rearrangement). May also occur with other amino acids at the amino terminal of a protein. It takes place on heating or prolonged storage and is one of the deteriorative processes that take place in stored foods. It is accompanied by a loss in nutritional value, since the amino acid that reacts with the sugar is not available.